In one conventional control system for a multiphase electric motor, for example, JP-9-312993A, a voltage of a rectangular waveform is modulated in pulse width when the voltage of the rectangular waveform is applied to a three-phase electric motor through 120° energization method (system). In this system, the pulse width modulation (PWM) is conducted in synchronism with the changeover timing of switching for applying the voltage of the rectangular waveform. This suppresses noises when changing over a switching element which conducts the pulse width modulation.
When the PWM processing is conducted in the above manner, the neutral point potential of a brushless motor changes in synchronism with the pulse width modulation. On the other hand, because the neutral point of the electric motor is generally disposed adjacent to a conductor through an insulator, the neutral point potential is equivalently grounded through a capacitor. In this case, when the neutral point potential changes in synchronism with the pulse width modulation, a current flows in the conductor side through the insulator from the neutral point, and the current may become a noise.
In another conventional control system for a multiphase electric rotating machine, when a three-phase electric motor is driven in a sensorless manner, an energization process from one specific phase to another phase is conducted twice while changing the phase so as to acquire an initial value of the rotation angle, to thereby fix the rotation angle. Even if the rotation angle of a rotor before the energization process starts is set to an uncontrollable angle which is close to an electric angle 180° (dead point) with respect to the final rotation angle because the rotation angle is thus controlled to the final rotation angle through two energization processes, the rotation angle of the electric motor can be controlled to the final rotation angle. That is, when the rotation angle of the rotor before the energization process starts is close to the dead point, the rotor cannot be changed by the energization process for controlling the rotation angle to the final rotation angle. However, the energization process is conducted twice, thereby making it possible to control the rotation angle of the electric motor to the final rotation angle.
In the energization processes of two times, for example, JP 3244800 (U.S. Pat. No. 5,396,159) proposes a technique in which the respective energization process times are set so that a frequency f1 (½×(processing time)) of the first processing, a frequency f2 (½ (processing time)) of the second processing, and a natural frequency of the electric motor satisfy a relationship of f1>F0>f2. As a result, the electric motor can be surely rotated forwardly when the electric motor starts.
A convergence time required when the electric motor is energized to converge the rotation angle to a given angle depends on inertia of the electric motor and a friction between the rotor and a bearing. That is, the convergence time is longer as the inertia is larger or the friction is smaller. For this reason, when the first energization process time is set based on a natural frequency in the above example, it is likely that the rotation angle of the start time point of the second energization process becomes an uncontrollable angle which is close to the dead point depending on the electric motor. In this case, the electric motor cannot appropriately start.
In a further conventional control system for a multiphase electric rotation machine, a rotor position is detected by detecting an induced voltage which appears in the terminal voltage of the stator windings without any rotor position detector, for example, a Hall element. For example, when a three-phase brushless DC motor is driven through a 120° energization method, a position signal is detected based on a comparison of the terminal voltage of the open phase with a reference voltage. In this case, in order to control the voltage applied to the motor and a current that flows in the motor, pulse width modulation control or current limit control is conducted.
Ringing (cyclic fluctuation) occurs in the terminal voltage when the energization to the stator windings changes from off-state to on-state under the PWM control or the current limit control. When the ringing occurs in the terminal voltage, a phase displacement (variation in time) occurs in the position signal that is provided by comparison of the terminal voltage with the reference voltage, resulting in rotation irregularity, noises, or step-out of phase.
JP 3,308,680 therefore proposes to latch a comparison result signal of the terminal voltage and the reference voltage at a down timing from the on-state to the off-state of the PWM signal. Therefore, the position signal can be provided without being affected by ringing which occurs in the terminal voltage. However, it is necessary to add a latch circuit to a microcomputer or a logic circuit which has been applied up to now as a new function circuit, or to accommodate with a latch circuit that is one of resources which are equipped in a microcomputer from another intended purpose. As a result, the circuit is complicated.